Based on the Sekai-ichi Hatsukoi ~Onodera Ritsu no Baai~ manga by Nakamura Shungiku (Junjou Romantica).

Related:

Sekai-ichi Hatsukoi OVASekai-ichi Hatsukoi TV Season 2

Summary

When Ritsu was in high school, he fell in love with a boy who broke his heart, changing his view of love forever. Now a jaded adult, Ritsu has just transferred to the Marukawa Shoten company in hopes of publishing literature, only to be told that he'll be working in the shoujo manga division instead. What's worse, his boss is brash, rude and seems to have an interest in him! However, the biggest surprise is yet to come for Ritsu, as he soon discovers that his boss is none other than his lover of his past! (Taken from Anime-Planet)